Solid-state additive manufacturing of 420 stainless steel using additive friction stir deposition

Additive Friction Stir Deposition (AFSD) is an emerging method for metal additive manufacturing (AM) that uses high shear strain below the alloy melting point to create refined microstructures and wrought mechanical properties in large-scale depositions. The present study is the first successful attempt to use AFSD to produce bulk 420 stainless steel deposits. Detailed microstructural characteristics of the deposited material were investigated to understand the micro-mechanisms operating during AFSD. The deposited microstructure had extremely fine martensitic laths, high dislocation densities, and intergranular M 7 C 3 /M 23 C 6 precipitates. Influenced by the thermal cycles of the process and significant deformation of parent γ grains, the microstructure showed different characteristics along the AFSD build direction: the first deposited layer had smaller parent γ grains and fine martensite laths for (i.e., ∼9μm and ∼130nm) when compared to the last deposited layer (i.e., ∼33μm and ∼230nm). The AFSD deposit showed a significant increase in average Vickers hardness, reaching up to 677 ± 33 HV. The increase was found to be mainly due to fine carbide precipitation and significant microstructure refinement. However, differences in the degree of martensite lath refinement with build height and interlayer WC tool wear debris caused hardness anisotropy across distinct layers and interlayers of the AFSD 420 deposit. These findings reveal the capacity of the AFSD to AM high-strength alloys with refined microstructures and attractive mechanical properties in the as-printed state.